RF switches are a key building block in wireless systems and find many uses in applications such as mobile phones and wireless LANS. The act of switching radio frequency signals in an integrated circuit is carried out by an RF switch circuit. Many technologies exist for the design of integrated RF switch circuits.
Generally an RF switch system does not consist of the RF switching circuit alone and typically must be controlled using digital logic circuitry. In addition both negative and positive voltage levels are required to optimise the performance of the switch. Therefore, a negative voltage generator may also be required as part of the switch system. A switch can have a number of switching elements, although it can be comprised of any number of switching elements, which control the flow of RF power between different points in the particular application. Often MOSFET switches are used as the switching element in the art of RF design.
Performance metrics such as low insertion loss, high linearity, high isolation and power handling are critical in RF switch design. However, the RF switch performance is limited by substrate interactions with the circuits which degrade all the key performance metrics as well as non-linearities in the circuits themselves. Switching to an SOI substrate has been shown to improve the loss in the substrate although additional treatment of the silicon substrate is necessary to improve linearity. This has been achieved using an implant step in the process which prevents charge build-up at the silicon surface beneath the oxide. However, these steps are still not sufficient to meet the stringent demands of wireless technologies and further techniques are required to serve the broader wireless market segment.
A publication by Botula A et al entitled ‘A Thin Film SOI 190 nm CMOS RF Switch Technology, IEEE, Piscataway, N.J., USA, 19 Jan. 2009, XP031415569 discloses a thin film SOI technology developed for RF switch applications and provides a buffer to protect the circuits from the RF signals. Other publications include WO2008/057524 assigned to Skyworks Solutions Inc., US2003/090313, Burgener et al, and US 2005/176184, Okihara Masao. However, none of the RF designs disclosed in these publications deal with the problem of unintended interactions of the RF and/or DC (or control) signals which are present, by injection of RF signals into the DC and low frequency analog blocks or by direct coupling of the wires on an IC chip. If a portion of the RF signal becomes inadvertently superimposed on the DC bias signals it degrades the linearity of the circuits making them unsuitable for wireless applications that demand low harmonic distortion.
One solution to the substrate interaction problem involves using an alternative substrate material such as sapphire. However, this significantly increases manufacturing costs and may not completely solve the interaction of RF and DC signals. Another solution involves the use of III-V technologies such as GaAs but this also increases costs and decreases integration levels.
There is therefore a need to provide a RF switch system, method and architecture to overcome the above mentioned problems.